The interaction of light with plasmonic sub-wavelength structures such as a single sub-wavelength aperture or array of sub-wavelength apertures in a metal or semiconductor film has been of interest due to the unique optical properties these structures possess. For example, Ebbesen et al. (U.S. Pat. No. 6,052,238, 18 Apr. 2000) teaches that an array of periodic sub-wavelength apertures can serve as a novel sub-diffraction optical element that has extraordinary optical transmission (EOT) properties and is capable of generating high electric near-fields in the vicinity of each aperture. Ebbesen further teaches that these phenomena happen because the array of sub-wavelength apertures permits the incident light to couple to Surface Plasmons (SP) and resonantly transfer through the sub-wavelength apertures to the other side of the metal or semiconductor film. Kim et al. (U.S. Pat. No. 6,285,020, 4 Sep. 2001) also teaches that fabrication of these devices often leads to a mismatch in the SP resonances between the top and bottom surfaces of the metal or semiconductor film due to a mismatch of the dielectric material used above and below the metal or semiconductor film. Matching of the dielectric material is possible, but this limits the selection of the dielectric to materials that can act as a structural support for the metal or semiconductor film, or to materials that have an index of refraction that is closely matched to the underlying dielectric.
Research has been performed to optimize the EOT and the electric near-field intensity of sub-wavelength aperture arrays in relation to the composition of the metal or semiconductor film, the geometry of the aperture, and the existence of structures nearby the apertures. Przybilla et al. (J. Opt. A: Pure Appl. Opt. 8, 2006) teaches that sub-wavelength aperture arrays in a noble metal have higher EOT compared to other metals. Lesuffleur et al. (J. Phys. Chem. C 111 (6), 2007) teaches that a sub-wavelength hole with a sharp apex double-hole structure produces a higher electric near-field intensity compared to two holes separated by a short distance. Gordon et al. (Opt. Express 15, 2007) teaches that Bragg reflectors consisting of corrugations surrounding the sub-wavelength hole array increase the transmission at the resonance peak due to back reflection of the SP waves within the region of the sub-wavelength aperture array.
Ebbesen et al. (Nature 391, 1998) and Krishnan et al. (Opt. Communications 200, 2001) teach that SP modes of sub-wavelength aperture arrays are dependent on the scattering orders of the apertures and dielectric properties of the materials on the top and bottom of the metal or semiconductor film. Krishnan et al. additionally teach how the optical resonance peaks related to sub-wavelength aperture arrays can be controlled by deposition of materials with a refractive index either below, equal to, or greater than the refractive index of the substrate such as Quartz. Krishnan et al. further teach that materials with the same dielectric constant in contact with the top and bottom surfaces of the aperture array in the metal film result in the coincidence of the SP resonance energies for SP modes on both surfaces of the metal film. de Dood et al. (Phys. Rev. B 77, 115437, 2008) teach how the optical transmission spectra of a sub-wavelength aperture array can be manipulated by changing the refractive index of a liquid material above the array relative to the material in contact with the underside of the array. The matching of SP resonance energies increases the EOT by a factor of 10 or more. Also, the electric near field intensity in the vicinity of the apertures will be significantly increased at the resonance wavelengths for the aperture array.
Each of these optimization methods have aimed to improve the performance of sub-wavelength hole arrays for applications such as biological and chemical sensing, Surface Enhanced Raman Spectroscopy (SERS), non-linear optics, super-lensing, optical filtering, and nanolithography.
Generally, sub-wavelength aperture arrays have been fabricated on solid substrates such as glass, quartz, Pyrex™ (Pyrex 7740 from semi wafer Inc.), polymer, SU8, or so forth due to the delicacy of the thin metal or semiconductor film. Therefore, matching of the dielectric properties of the top and bottom surface of the sub-wavelength aperture array in the metal or semiconductor film has been limited to materials with dielectric properties similar to the substrate that can be deposited on to the film. Also, each of the index matching approaches to fabrication may have one or more limitations. As Yang et al. teaches (Nano. Lett. 8, 2008), there is not always a good dielectric matching between the refractive indices of various biological solutions and the substrate of the sub-wavelength aperture array. Based on this fact, the interference of the resonance peaks with the same mode from the substrate and biological solution could result in poorer sensitivity. Furthermore, with current fabrication methodologies it is not possible to dynamically change the material below the metal or semiconductor film due to the presence of the substrate material. For example, a sub-wavelength aperture array device on a glass substrate will display resonance peaks related to the interface between the glass substrate and the metal or semiconductor film as well as the resonance peaks related to the interface between the top side material and the metal or semiconductor film. The resonance peaks from top side of sub-wavelength aperture arrays can be tuned by selecting a top side material with an appropriate refractive index. However, the resonances from the substrate side cannot be changed. Therefore, in order to match the resonances with the same mode from both the top and bottom side of a sub-wavelength aperture array, one would have to select a material with the same refractive index as the substrate for deposition on top of the sub-wavelength-hole arrays in order to gain high transmission at the resonance peaks. Also, some materials like gases such as air have a refractive index close to one and there is no suitable substrate material available with a similar refractive index. Therefore, refractive index matching may not be readily accomplished for a wide range of materials using current sub-wavelength hole array fabrication methodologies.
Other difficulties with existing systems may be appreciated in view of the detailed description herein below.